When organisms from two different species reproduce, their offspring are known as hybrids. Postzygotic barriers are mechanisms that activate after a hybrid zygote, or fertilized egg, has formed. These barriers prevent the hybrid from developing into a healthy, fertile adult, thus stopping the flow of genes between the two parent species. This process helps to keep species distinct over evolutionary time.
It is as if two different sets of building instructions were merged; while a structure might begin to form, the conflicting directions ultimately prevent the successful completion of a functional product. These mechanisms act as a genetic safety net, ensuring that hybrid offspring are unable to pass on their mixed genetic information.
The Role in Reproductive Isolation
To understand postzygotic barriers, one must first grasp reproductive isolation, the collection of all processes that prevent members of two different species from successfully interbreeding. These isolating mechanisms are divided into two main categories based on when they occur in the reproductive cycle. The first, prezygotic isolation, includes all barriers that prevent mating or fertilization from ever happening.
Examples of prezygotic barriers are differences in mating seasons, incompatible reproductive organs, or distinct courtship rituals not recognized between species. These barriers are the first line of defense in keeping species separate.
The focus of this article is on postzygotic isolation, which takes effect after fertilization has occurred, acting on the hybrid zygote itself. Postzygotic barriers ensure that this genetic combination does not lead to a new, viable lineage that could blur the lines between the parent species.
Hybrid Inviability
Hybrid inviability is a direct postzygotic barrier where the hybrid zygote is unable to develop into a living organism. The genetic instructions from the two parent species are often incompatible, leading to critical errors during embryonic development. These genetic conflicts can disrupt fundamental cellular processes, causing the embryo to fail at an early stage.
For example, when certain species of frogs are crossed, the resulting embryos may not survive. Crosses between the African clawed frog (Xenopus laevis) and the Western clawed frog (Xenopus tropicalis) demonstrate this. While one cross is viable, the reciprocal cross results in embryos that die very early in development due to a mismatch in developmental signals.
Similar issues are observed in salamanders of the genus Ensatina. These salamanders form a “ring species” where adjacent populations can interbreed, but the populations at the ends of the ring are too genetically different. When these terminal forms mate, their hybrid offspring are often inviable.
Hybrid Sterility
Hybrid sterility occurs when a hybrid offspring develops into a healthy adult but cannot produce functional gametes—sperm or eggs. The most widely recognized example is the mule, the offspring of a male donkey and a female horse. Mules are known for their strength and are healthy animals, but the issue arises at the chromosomal level.
Horses have 64 chromosomes, while donkeys have 62. A mule inherits 32 chromosomes from its horse mother and 31 from its donkey father, resulting in a total of 63 chromosomes. This odd number creates a problem during meiosis, the process of creating sex cells.
Meiosis requires chromosomes to pair up before they are divided into gametes. With an uneven number, the chromosomes cannot form proper pairs, preventing the formation of viable sperm or eggs. This chromosomal mismatch is a common cause of sterility in hybrids, including ligers (offspring of a male lion and a female tiger).
Hybrid Breakdown
A more subtle postzygotic mechanism is hybrid breakdown. In this scenario, the first-generation (F1) hybrids from an interspecies cross are both viable and fertile. The problem emerges in the next generation (F2), produced when F1 hybrids mate with each other or with one of the parent species.
The F2 generation often suffers from reduced fitness, appearing weak or sterile. This occurs because while the initial combination of genes in F1 hybrids is functional, the reshuffling of these genes during the F1 generation’s gamete formation creates new, incompatible gene combinations in the F2 generation.
This phenomenon is frequently observed in the plant kingdom. For instance, when certain cultivated rice varieties (Oryza sativa) are crossed, the F1 generation is robust and fertile. However, the F2 offspring often exhibit problems like weakness and sterility. Similar effects have been documented in insects, such as different populations of the fruit fly Drosophila melanogaster.
Evolutionary Significance
Postzygotic barriers play a significant part in evolution by maintaining the distinctness of species. When two populations diverge genetically, these barriers prevent them from merging back together if they come into secondary contact. By ensuring that hybrid offspring are inviable, sterile, or have reduced fitness, these mechanisms effectively stop the flow of genes between the diverging species.
The existence of postzygotic barriers can also drive the evolution of prezygotic barriers in a process known as reinforcement. If hybrid offspring are unfit, natural selection will favor individuals that avoid mating with members of the other species. This prevents them from wasting reproductive effort on non-viable offspring and strengthens prezygotic barriers, making interspecies mating less likely.
Postzygotic barriers are not just a consequence of genetic divergence but also a driver of further reproductive isolation. They help solidify the boundaries between species, ensuring they remain on separate evolutionary trajectories.